Conventionally, various optical three-dimensional measurement methods applied to a rough surface object are known as a method for measuring a shape of an object surface by non-contact. For example, some methods are known which use a lattice image distorted by a target object from an original lattice pattern projected onto the object with a projector. Followings are included in these methods: a moire method taking a picture of the lattice image as a moire image (contour line); a phase shifting method taking a picture of the lattice image by changing the phase of light; and a spatial code method deriving space coordinates for groups of points distributed on a surface from focus locations in an image. Furthermore, a spot light method which takes an image of spots on an object illuminated with light beams and derives their space coordinates from focus locations in the image, and a light-section method which illuminates an object with a slit light and derives a series of points of space coordinates using focus locations in an image of a belt-like light distorted on the object are known. In addition, there is a stereo method using two pictures taken from two directions centering on the object. However, these measurement methods mainly measure a static object, and therefore there is a limit to highly precise three-dimensional shape measurement for a moving object. Moreover, these methods use an image formation lens for a projector or a photographing optical system (camera), therefore measurement errors caused by distortion of image or focal gap, etc. occur and an accuracy of three-dimensional measurement is limited.
In recent years, three-dimensional range finding cameras etc. using TOF method are presented which can measure distances to an object via phase differences between an original modulated signal and a returned light's modulated signal, where the returned light is obtained, first, emitting an intensity modulated light of a pulse laser light or a CW laser light to the object and, second, recording a scattered-reflected light from the object surface at a rate of 30 frames per second. However, its distance measurement accuracy is about 1 cm, and there are problems about such as speeding up of pulse-making or intensity-modulation of CW at light source, and speeding up of a signal processing circuit, etc. in order to realize accuracy of 1 mm or less.
Moreover, an interference measurement method for a shape of rough surface using holography is studied in the prior art. The holography is technology which can record an object light wave-front and reconstruct the light wave-front or an image in three-dimensional space. The interference measurement method requires two sheet holograms for generating an interference fringe pattern, and therefore a measuring target is fundamentally restricted to a static object. Moreover, the interference measurement method usually uses interference fringe pattern analysis or phase connection. The phase connection tends to cause errors under the influence of speckles. The speckles arise in the interference measurement for a shape of rough surface. Furthermore, the contrasts in the observed interference fringes deteriorate upon coming off the object surface and localize near the object surface. The speckles and the localization of the interference fringes cause errors easily in the phase connection of the interference fringe pattern. In other words, the interference fringe pattern analysis and phase connection indispensable for the interference measurement method tend to cause errors for an object with large depth or an object of a complicated shape with discontinuity surface, and therefore this interference measurement method is not suitable for high precision three-dimensional measurement.
Incidentally, there is a method called as a shape-from-focus method for obtaining an object shape using contrasts in object images recorded with an image sensor. The shape-from-focus method determines the object surface position by searching an in-focus position (a focusing point) of an object surface from among the recorded images which are recorded by changing the focal distance of a lens. However, it is necessary to record a plurality of images of different focal distances at once to perform three-dimensional measurement of a moving object by the shape-from-focus method. Thus, as a method for realizing dynamic shape measurement of a micro component, a shape-from-focus method using holography is proposed which takes advantage of the free focal image reconstruction of holography (for example, refer to patent document 1).